THIS INVENTION relates to a seal arrangement for effecting a seal between a movable rod, shaft or like element and a passage through which said element extends, for example for sealing a shaft with respect to a bore in which the shaft is movable axially, in a pneumatic system.
One of the key factors in the design of pneumatic systems with sliding elements is the design of the sealing system. Over the years a very large number of such systems have been adopted but all have ultimately been a compromise between achieving a high level of seal integrity and a low level of friction. Considerations of space, cost, tolerance to misalignment, wear rate and required manufacturing tolerances of the containing parts have been other important elements.
The increasing move towards the use of non lubricated air in pneumatic systems has emphasised the problem of achieving low friction, in order to reduce wear and to obtain other advantages associated with low friction.
Known sealing systems have used a range of designs and employed a variety of materials, combinations of materials and lubricants. One of the simplest seal arrangements is the elastomeric xe2x80x98Oxe2x80x99 ring and this is still widely used for both static and dynamic sealing. Used in the conventional mode the xe2x80x98Oxe2x80x99 ring is installed so that it is under constant radial compression. This gives good sealing but because of the tolerances involved and the way in which it is loaded pneumatically it tends to suffer from high wear rates and be intolerant to misalignment.
An alternative method of using an xe2x80x98Oxe2x80x99 ring is to use it in the so-called xe2x80x98floatingxe2x80x99 mode. In this mode the ring is installed with a nominal interference with the sliding surface but with radial clearance in the groove, receiving the xe2x80x98Oxe2x80x99 ring, in the member which slides relative to that sliding surface. In this mode the pressure exerted by the ring on the sliding surface is much reduced and hence so is the friction. The problem with this arrangement is that sealing can now only be achieved if the xe2x80x98Oxe2x80x99 ring is in contact with the groove wall which is exposed to the lower pressure of any differential across the seal. In some designs it is not difficult to ensure that this is so but in many applications it is possible for conditions to arise which lift the seal off that groove wall. This is particularly true during start up conditions when pressure is first applied. Once a flow is established around under the xe2x80x98Oxe2x80x99 ring there is no pressure force to push it towards the respective groove wall to establish sealing. It is normal, therefore, to limit the axial clearance between the groove and the xe2x80x98Oxe2x80x99 ring section to a dimension below which the ring becomes essentially bistable. At the opposite extreme it is not possible to use too small an axial clearance between the xe2x80x98Oxe2x80x99 ring and the groove because in this case, when pressure is first applied during start-up, the pressure forces the ring down into the groove and once flow is established over the top of the ring sealing cannot be restored.
The net result of these limitations is that, to be sure of reliable operation of floating xe2x80x98Oxe2x80x99 rings, clearances must be held to a tolerance of the order of 0.05 mm. International Standards for xe2x80x98Oxe2x80x99 ring dimensions only require the section of smaller xe2x80x98Oxe2x80x99 rings to be within xc2x10.08 mm. Therefore, selection of xe2x80x98Oxe2x80x99 rings and matching with grooves is necessary to ensure that a floating xe2x80x98Oxe2x80x99 ring operates satisfactorily. Even with this care, additional attention has to be paid to the amount of grease applied as too much on the side of the ring can create the same effect as too narrow a groove.
It is an object of the present invention to provide an improved seal arrangement which avoids the above-noted disadvantages.
According to one aspect of the invention there is provided a seal arrangement providing a seal between a passage and a rod or like element movable in said passage, the seal arrangement including a floating elastomeric sealing ring, as herein defined, which is accommodated in a circumferential groove around said rod or like element, and which sealingly engages the wall of said passage, and an annular resilient biasing element also accommodated in said groove, the biasing element acting between one wall of said groove and the sealing ring and serving to urge the sealing ring against the opposing wall of said groove, the biasing means being configured to allow the passage of fluid past it between the region of the groove disposed radially inwardly of the sealing ring and the gap between said rod and passage on the side of the sealing ring opposite said opposing wall.
According to another aspect of the invention there is provided a seal arrangement providing a seal between a passage and a rod or like element movable in said passage, the seal arrangement including a floating elastomeric sealing ring, as herein defined, which is accommodated in a circumferential groove around said passage and which sealingly engages the surface of said rod or like element, and an annular resilient biasing element also accommodated in said groove, the biasing element acting between one wall of said groove and the sealing ring and serving to urge the sealing ring against the opposing wall of said groove, the biasing means being configured to allow the passage of fluid past it between the region of the groove disposed radially outwardly of the sealing ring and the gap between said rod and passage on the side of the sealing ring opposite said opposing wall.
According to a further aspect of the invention there is provided a seal arrangement providing a seal between a passage and a rod or like element movable in said passage, the seal arrangement including an elastomeric ring which is accommodated in a circumferential groove around said rod or like element and which sealingly engages the wall of said passage and one wall of said groove, but has a radial clearance with respect to the bottom of said groove, said ring being compressed axially between the opposing walls of said groove and being configured, on its side engaging the groove wall opposite said one wall, so as to provide at least one passage for fluid flow between, on the one hand, the gap between said rod and said passage, and, on the other hand, the region between the ring and the bottom of said groove.
The invention allows the advantages of xe2x80x9cfloatingxe2x80x9d xe2x80x98Oxe2x80x99 rings or similar xe2x80x9cfloatingxe2x80x9d sealing rings to be achieved without most of the disadvantages.
The invention makes it possible to maintain the xe2x80x98Oxe2x80x99 ring or other sealing ring in contact with the lower pressure wall, irrespective of movement or actual pressure differential, whilst still ensuring that the ring can be energised against the sliding surface by any applied pneumatic pressurexe2x80x94however suddenly applied.